1. Field of the Invention
This invention relates generally to metallurgical testing of weld joints and more particularly, to a destructive tensile test to evaluate a tubular weld joint.
2. Related Art
Where a number of different welds are being applied under identical conditions such as on an assembly line, it is desirable to periodically take a sample to test the quality of the weld. Radiological and ultrasonic nondestructive techniques have been employed in the art in the past for this purpose. Where the integrity of the weld is important for the application, especially where safety considerations are involved, it is desirable to perform a more exacting test on the periodic samples to confirm the integrity of the metallurgical bond. Such is the case in the manufacture of nuclear fuel rods formed from a tubular cladding that is welded to an end cap. In the case of a nuclear fuel rod, it has not been practical to perform a tensile fracture test to test the weld, because the weld microstructure has a higher strength than the cladding. As a result, tensile fracture of the unsupported cladding tube weld specimen is always in the tube region away from the weld. Such a fracture location is unable to capture non-bond regions of the weld (if they were to exist) on the fracture surface. Introduction of a notch at the weld bond line on the tube outer surface to induce failure in the weld is cumbersome and does not always consistently localize fracture in the weld region. For that reason, current practice is to employ a metallographic evaluation of weld bonding which provides weld bonding data for a single cross section of the weld and not the entire circumference. Metallographic examination of weld specimens involves specimen sectioning, polishing, acid etching and disposal of the acid waste. The steps for metallographic evaluation require significant effort, expense and time.
Accordingly, it is an object of this invention to provide a method and apparatus that provides a faster and less costly examination of a weld""s metallurgical structure on a tubular member that distinctively contrasts well bonded regions from unbonded regions in the weld. It is a further object of this invention to provide such an apparatus and method that examines the entire weld circumference.
These and other objects are achieved by the method and apparatus of this invention which provides a tensile test for a tubular member that initiates failure in a circumferential weld. The tube region immediately on one side of the weld is completely supported by a mandrel and the specimen is gripped on either side of the weld along the longitudinal axis of the tube. In the preferred embodiment, a very small separation distance is maintained between the mandrel supporting the tube region on one side of the weld and the gripping mechanism supporting the specimen on the other side of the weld. A force is applied to one or both of the tensile grips in a direction away from the weld along the longitudinal axis of the tube to initiate failure in the weld. If there are any non-bond defects in the weld, they are captured by the weld fracture surface as mesa or void features with an orientation different from that of a fracture of a well bonded region. The fracture surface orientation for a well bonded region is approximately 45xc2x0 to the tensile axis, while the fracture orientation of a non-bonded region is approximately perpendicular to the tensile axis. The degree of non-bonding is calculated from the area fraction of fracture surface area showing the distinct non-bond features as revealed by the examination of the fracture surface under a stereomicroscope. The tensile test of this invention evaluates weld bonding over the entire weld circumference and provides weld bonding results on production weld evaluation specimens quickly and with less effort than is required to perform a metallographic examination.